Intracellular gelation of deoxyhemoglobin S is believed to cause the sickling of SS erythrocytes. We use 13C/1H double nuclear magnetic resonance (NMR) to quantitate the amount of intracellular polymer formed as a function of oxygen saturation. We found that for SS erythrocytes polymer could be detected at high oxygen saturation (greater than 90%) and increased with decreasing oxygen saturation, in excellent agreement with theoretical calculations based on the high intracellular hemoglobin S concentration (34 g/dl) where non-ideality effects must be considered. We are now fractionating the whole cell population to characterize the effect of cell heterogeneity on intracellular polymerization. In studies of the other sickle syndromes we found that for AS erythrocytes, polymer was detected below 60% oxygen saturation, increasing to 0.4 of the total hemoglobin at complete deoxygenation. This is in agreement with polymer fraction derived on the basis of AS hemolysate solubility measurements. We are using 31 PNMR to obtain additional information about intracellular polymerization such as the extent of ligand incorporation in the polymer. In general, these studies will be useful for evaluation of therapeutic. In general, these studies will be useful for evaluation of therapeutic strategies.